High Performance Thin Films for Microwave Phase Shifter Applications: Device Requirements, Material Design, and Process Science Considerations

Abstract

Ba (1-x) Sr x TiO 3 (BST) thin films appear to be excellent candidates for microwave phase shifters. However, the major issue impeding the utilization of BST films in beam steering devices is the simultaneous achievement of the required performance properties. These properties, at frequencies > 10 GHz, include a dielectric permittivity ≤500, low dielectric loss (<2%), high tunability (-2:1), low leakage current characteristics, and low operating control voltages (<10V). Additionally, these properties must be obtained via an industry standard growth and processing methods. Our approach to satisfy the above mentioned dielectric properties of BST based films is focused on material compositional design and optimized film processing parameters. A systematic study was employed to examine the influence of low concentration Mg acceptor doping and optimized post-deposition annealing conditions on the structure and microwave dielectric properties of BST based thin films fabricated via the metalorganic solution deposition technique. The Mg doping was found to have a strong influence on the material properties of the BST films whereby the films permittivity, dissipation factor, and leakage characteristics were significantly reduced with respect to that of undoped BST. Optimum material properties were achieved for the 5 - 7 mol% Mg doped BST films. At these doping concentrations the dissipation factor (10 GHz) for both films was less than 1.7%. The leakage characteristics were 5.78x10 -8 and 7.97x10 -9 , respectively, and the tunability was ∼40%. Our results suggest that Mg doping and optimized post-deposition annealing served to mitigate the oxygen vacancies thereby lowering the films dielectric loss. Furthermore, the acceptor doping maintained the dielectric permittivity well below 500. Our results suggest the 5-7 mol% Mg doped BST thin films to be commercially viable for microwave phase shifter devices.